char: change qemu_chr_fe_add_watch to return unsigned

g_source_attach can return any value between 1 and UINT_MAX if you let
QEMU run long enough.  However, qemu_chr_fe_add_watch can also return
a negative errno value when the device is disconnected or does not
support chr_add_watch.  Change it to return zero to avoid overloading
these values.

Fix the cadence_uart which asserts in this case (easily obtained with
"-serial pty").

Tested-by: Bret Ketchum <bcketchum@gmail.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/char/cadence_uart.c

@@ -292,9 +292,12 @@ static gboolean cadence_uart_xmit(GIOChannel *chan, GIOCondition cond,
     memmove(s->tx_fifo, s->tx_fifo + ret, s->tx_count);
 
     if (s->tx_count) {
-        int r = qemu_chr_fe_add_watch(s->chr, G_IO_OUT|G_IO_HUP,
-                                      cadence_uart_xmit, s);
-        assert(r);
+        guint r = qemu_chr_fe_add_watch(s->chr, G_IO_OUT|G_IO_HUP,
+                                        cadence_uart_xmit, s);
+        if (!r) {
+            s->tx_count = 0;
+            return FALSE;
+        }
     }
 
     uart_update_status(s);

include/sysemu/char.h

@@ -221,8 +221,20 @@ void qemu_chr_fe_event(CharDriverState *s, int event);
 void qemu_chr_fe_printf(CharDriverState *s, const char *fmt, ...)
     GCC_FMT_ATTR(2, 3);
 
-int qemu_chr_fe_add_watch(CharDriverState *s, GIOCondition cond,
-                          GIOFunc func, void *user_data);
+/**
+ * @qemu_chr_fe_add_watch:
+ *
+ * If the backend is connected, create and add a #GSource that fires
+ * when the given condition (typically G_IO_OUT|G_IO_HUP or G_IO_HUP)
+ * is active; return the #GSource's tag.  If it is disconnected,
+ * return 0.
+ *
+ * @cond the condition to poll for
+ * @func the function to call when the condition happens
+ * @user_data the opaque pointer to pass to @func
+ */
+guint qemu_chr_fe_add_watch(CharDriverState *s, GIOCondition cond,
+                            GIOFunc func, void *user_data);
 
 /**
  * @qemu_chr_fe_write:

net/vhost-user.c

@@ -22,7 +22,7 @@ typedef struct VhostUserState {
     NetClientState nc;
     CharDriverState *chr;
     VHostNetState *vhost_net;
-    int watch;
+    guint watch;
     uint64_t acked_features;
 } VhostUserState;
 

qemu-char.c

@@ -3966,19 +3966,19 @@ void qemu_chr_fe_event(struct CharDriverState *chr, int event)
     }
 }
 
-int qemu_chr_fe_add_watch(CharDriverState *s, GIOCondition cond,
-                          GIOFunc func, void *user_data)
+guint qemu_chr_fe_add_watch(CharDriverState *s, GIOCondition cond,
+                            GIOFunc func, void *user_data)
 {
     GSource *src;
     guint tag;
 
     if (s->chr_add_watch == NULL) {
-        return -ENOSYS;
+        return 0;
     }
 
     src = s->chr_add_watch(s, cond);
     if (!src) {
-        return -EINVAL;
+        return 0;
     }
 
     g_source_set_callback(src, (GSourceFunc)func, user_data, NULL);